Circuit breakers, in particular for low voltages, are known and are also used for direct-current operation, and are connected on the outgoer side to at least one load. Below the rated current of the switch, a direct current or alternating current which corresponds to the load flows via the load resistance through a conductor which runs through the switch. When the switch is closed, contact elements rest on one another, and are disconnected from one another in order to open the switch. An electronic tripping unit initiates the disconnection of the contact elements when a predetermined current limit value is exceeded; this is the case when an overcurrent and a short-circuit current occur. In order to supply power to the tripping unit, a voltage is tapped off on the conductor, which voltage is dropped across the load resistance when the switch is closed, and across the switching contacts when the switch is open. Therefore, when the switch is closed, the voltage tap is in each case dependent on the presence of a load which is not excessively low.
When a short circuit occurs, the load and therefore the voltage drop across the load resistance become so small, however, that the power supply to the tripping unit is no longer ensured.